1. Field of the Invention
The present invention relates to a panoramic imaging lens capable of imaging a lateral perspective of 360xc2x0 circumference as an annular image.
2. Description of the Related Art
As a panoramic imaging lens that can image a perspective of 360xc2x0 circumference at one time, lenses disclosed in U.S. Pat. Nos. 4,566,763 and 5,473,474 have been conventionally known. The panoramic imaging lens 1 is formed with a light permeable material, such as optical glass, a transparent resin and the like, in a rotation symmetrical form around a light axis, which is a central axis of the lens as shown in FIG. 1. The panoramic imaging lens 1 has an annular light incident surface 2, on which lateral light from the 360xc2x0 full circumference is incident; a first reflective surface 3 formed in an annular form that is adjacent to and substantially opposes the light incident surface 2 to reflect the light inside the lens 1; and a second reflective surface 4 provided at a central part inside the annular light incident surface 2 to reflect the reflected light from the first reflective surface 3 inside the annular first reflective surface 3. A light outgoing surface 5 transmitting the light from the second reflective surface 4 is formed at a position opposing the second reflective surface 4 in a central part of the first reflective surface 3. The light incident surface 2 is formed in a form of a convex lens bulging laterally, and the second reflective surface 4 inside it is formed in a concave form, whereby the inner surface of the lens 1 is a reflective surface in a form of a convex mirror. The first reflective surface 3 adjacent to the light incident surface 2 is also formed in a form of a convex surface bulging laterally, and the inner surface of the lens 1 is an annular reflective surface of a concave mirror form directed to the second reflective surface 4. The light outgoing surface 5 is appropriately formed in a concave or convex surface or a planar surface.
A panoramic imaging system using the panoramic imaging lens 1 comprises, as shown in FIG. 1, a panoramic imaging adapter 10 having the panoramic imaging lens 1 and a relay lens 6 for image formation provided on the light axis, and an imaging device 8, such as a video camera and the like, to which the panoramic imaging adapter 10 is attached. The imaging device 8 comprises a lens system 9 for acquiring an image, an imaging element 7, such as a CCD and the like, provided at the image focus location thereof, and a necessary controlling sections and the like not shown in the figure.
A panoramic landscape 11 of 360xc2x0 circumference acquired by the panoramic imaging lens 1 is converted to an electric signal by the imaging element 7 of the imaging device 8 and projected on various kinds of a monitoring device 14, such as a liquid crystal, a CRT and the like.
With respect to the panoramic imaging lens 1, in the case where the panoramic imaging adapter 10 is arranged in such a direction that the light incident surface 2 turns upward as shown in FIG. 1, the lowermost light L1 from the panoramic landscape 11 of 360xc2x0 circumference amongst the incident light incident on the light incident surface 2 of the panoramic imaging lens 1 is refracted at the light incident surface 2, then reflected at the opposing first reflective surface 3 toward the second reflective surface 4, and further reflected at the second reflective surface 4 to outgo from the light outgoing surface 5, followed by being imaged through the relay lens 6 on a circumferential part of the imaging element 7. In the case where the uppermost light L2 from the panoramic landscape 11 amongst the incident light is incident on the light incident surface 2, it is reflected at the light incident surface 2 of the panoramic imaging lens 1, then reflected at the opposing first reflective surface 2 toward the second reflective surface 4, and further reflected at the second reflective surface to outgo from the light outgoing surface 5, followed by being imaged through the relay lens 6 on a central part of the imaging element 7. According to the foregoing mechanisms, the panoramic landscape 11 is imaged in an annular form on the imaging element 7 and projected as an annular two-dimensional image 12 on the monitoring device 14.
According to the foregoing conventional technique, there is such a problem that a part of the light incident on the light incident surface 2 of the panoramic imaging lens 1 is incident on the imaging element 7 as flare and ghost. The phenomenon of flare or the like is caused by mixing external noise light Ln functioning as flare or the like with the regular imaging light LO on the light path inside the lens as shown in FIG. 2A. The noise light Ln is incident on the light incident surface 2 of the panoramic imaging lens 1, and after reflected on the first reflective surface 3 and the second reflective surface 4 inside the panoramic imaging lens 1, most of the noise light Ln again outgoes from the light incident surface 2 as shown in FIG. 2A. However, a slight part of the noise light Ln1 amongst the noise light Ln is inwardly reflected on the light incident surface 2 inside the panoramic imaging lens 1 and proceeds to agree with the light path of the regular imaging light L0 in the lens. Therefore, the noise light Ln1, which is a part of the noise light Ln, is inwardly reflected on the light incident surface 2, and then it agrees with the light path of the regular imaging light L0 to reach the imaging element 7, and is projected as flare and ghost.
The invasion path of the noise light Ln includes, in addition to the foregoing, noise light Ln2 causing flare or the like that suffers repeated reflection inside the panoramic imaging lens 1 and then agrees with the incident path of the regular imaging light L0 by the inward reflection on the light incident surface 2 as shown in FIG. 2B. Moreover, as shown in FIG. 2C, noise light Ln3 causing flare or the like is included, which after reflected once inside,the panoramic imaging lens 1, agrees with the incident light path of the regular imaging light L0 by the inward reflection on the light incident surface 2.
With respect to the flare and ghost, partial strong light is particularly liable to function as the noise light Ln when light is incident on the panoramic imaging lens 1 from the 360xc2x0 circumference. In the case where the panoramic imaging system is used as a surveillance camera or the like, there is such a problem and the like that the flare and ghost adversely affect the control of light amount to deteriorate quality of the pictured image.
The invention has been developed in view of the foregoing problems associated with the conventional technique, and an object thereof is to provide a panoramic imaging lens capable of certainly suppressing noise light causing flare and ghost with a simple structure.
Specifically, the invention relates to a panoramic imaging lens comprising a light transmitting material, such as optical glass, a transparent resin and the like, and comprising an annular light incident surface formed in a rotation symmetrical form around a light axis, which is a central axis of the lens in a substantial convex lens form bulging to enable incident of lateral light from the 360xc2x0 full circumference; afirst reflective surface formed in an annular concave mirror form bulging outwardly that substantially opposes the light incident surface to reflect light inside the lens; a second reflective surface formed in a convex mirror form provided at a central part inside the annular light incident surface to reflect the reflected light from the first reflective surface toward an inner part of the annular first reflective surface; and a light outgoing surface positioned at a central part inside the annular first reflective surface and opposing the second reflective surface to transmit the light from the second reflective surface. The panoramic imaging lens is a panoramic imaging lens that is equipped with a non-reflective part exerting no regular reflection of light on a light path toward the light incident surface amongst light paths of light proceeding to agree with a light path of imaging light incident on the light incident surface and being refracted and proceeding inside the lens, which are light paths of light partly and inwardly reflected by the light incident surface from the interior of the lens toward the interior of the lens.
The non-reflective part of the invention may be a light absorbing part formed to be black or a dark color that absorbs light but does not reflect light, la diffused reflective part formed to be a roughened surface that diffusely reflects light but does not regularly reflect light, or a combination thereof.
The non-reflective part may be an opening having a form of a cylinder, a crucible former or the like that is formed in a rotation symmetrical form around a light axis of the panoramic imaging lens from the light outgoing surface toward the second reflective surface of the lens, and the opening has such a depth that a bottom of the opening is at a position on a side of the light outgoing surface with respect to a line connecting a fringe of the light incident surface on a side of the first reflective surface.
The non-reflective part may also be an opening having a form of a cylinder that is formed in a rotation symmetrical form around a light axis of the panoramic imaging lens protruding from the light outgoing surface toward a direction opposite to the second reflective surface.
The non-reflective part may also be formed on at least one of a periphery of the light outgoing surface and the first reflective surface of the panoramic imaging lens in an annular form with a light axis of the light outgoing surface as a center.
The non-reflective part may also be formed on a side peripheral surface of the panoramic imaging lens formed between the light incident surface and the first reflective surface of the panoramic imaging lens.